InfluxDB 2: Cannot add a existing label to an organization

```bash
--> POST http://127.0.0.1:9999/api/v2/orgs/05dc55ba177c2000/labels
--> {"labelID":"05dc55ba21536000"}

<-- 400 Bad Request http://127.0.0.1:9999/api/v2/orgs/05dc55ba177c2000/labels
<-- {
"code": "invalid",
"message": "label name is required"
}
```

__Expected behavior:__
Add label to an organization.

__Actual behavior:__
Produce `400 Bad Request` HTTP response.

__Environment info:__

* InfluxDB version: latest nightly build

cc @AlirieGray

Start by reproducing the POST request to /api/v2/orgs/{orgID}/labels with an existing labelID, then trace the organization-label API handling that returns “label name is required.” Done means an existing label can be added to the organization and the endpoint no longer returns 400.

Written by the indexing model from the issue text.